DiUS Build-Lights have been an essential part of our agile software development since its inception in 2014. It was originally hacked together for a project and quickly became popular. There were a few issues with the hardware so I ‘upgraded’ the hardware to version 2.0, hence DiUS Build-Lights 2.0. If you want to find out about the DiUS Build-Lights, read part one!

Electronics

The custom made PCB is a HAT that plugs on top of a Raspberry Pi. HAT is short for Hardware Attached on Top and is a form factor specified by the Raspberry Pi Foundation. The Build-Lights HAT takes 12V DC power, steps it down to 5V and powers the Pi and the LED strip. There is a level shifter that converts the Raspberry Pi’s 3.3V outputs to 5V for the LED strip.  I have also opto-isolated the serial port to make the hardware as flexible as possible. It sounds like a simple board, it should be easy right?

I started out with the schematic entry in Eagle CAD software. I carefully selected the components as I went along with the schematic entry. It went smoothly and I quickly proceeded to PCB layout without thinking much about the things that could go wrong. I was careless. I had forgotten about Murphy’s law - anything that can go wrong, will go wrong. I was faced with hardware problems on a simple board.

PCB fabrication


After finishing the PCB layout, I sent my design files to ITEAD Studio to manufacture the PCB. I chose ITEAD Studio because they make high quality PCB with 100% e-test on all the PCB’s at low cost. Their PCB turnaround time is around 1 to 2 weeks, which is pretty fast. The only complaint I have is the silkscreen on the green PCB’s as they are blurry as shown in the photo below, but that is not an issue on our more expensive black PCB’s.

Hardware problems

Two weeks later, I was excited to get my first batch of PCB’s from ITEAD Studio. I hand loaded all the components only to realise that the connectors that I had chosen were too small, too flimsy, and too fiddly to handle. It was the Molex Pico-Lock-Blade connector. There were other minor hardware problems too, like a wrong component footprint. The good news was, the hardware was working as intended, but it was only borderline working as I found out later. Disappointed, I hastily changed to another connector and sent the new design off to ITEAD Studio.

Another two weeks later, I received the PCB and totally expected it to work only to be disappointed again. This time, it was a different issue. The LED strip flashed random colours intermittently and it was difficult to track down the cause of the problem. Luckily I still remember my transmission line theory for high-speed digital signal from my university days. I managed to track it down to the unterminated high-speed SPI signals reflecting in the LED strip cable, and it was confusing the level-shifter. Adding a simple source termination resistor on each of the SPI signals solved the problem. How did the previous revision PCB ever work? The LED to Pico-Lock-Blade cable assembly was considerably shorter than the new cable, and the signal reflection was not as bad.

Laser cut acrylic enclosure

It was my first time designing a laser cut acrylic enclosure. I had doubts on the cost and the appearance of the final product. I found an open-source Raspberry Pi acrylic enclosure from www.pi-supply.com and I used it as a starting point for the Build-Lights enclosure. The design is just a simple vector graphics file with the 6 pieces to cut out. Having absolutely no experience in laser cutting, I randomly picked an online laser cutting service called Online Laser Cutting (OLC). They use Inkscape software, so I used Inkscape software as well to minimize compatibility issues.

I needed a way to quickly test my design, and printing it on a piece of paper and sticking it on a 3mm cardboard was a perfect way to test.

After a few iterations, I was ready to laser cut the acrylic. I sent my design to OLC and was surprised to get the cut acrylic back in 3 days. The cost was comparable to an off-the-shelf enclosure. The only problem was that the design was scaled up by 20%. OLC was kind enough to cut another copy for free, but with the correct scaling this time. Another 3 days later, I received the perfectly cut acrylic enclosure.

Summary

The DiUS Build-Lights started out as my pet project. Originally, I only wanted to design a simple HAT, but more requirements creeped in quickly and I ended up with a custom enclosure as well. It has been a great learning experience for me, and it is rewarding to see a nice looking device be created out of this whole exercise.